Wireless power transfer based on electromagnetic resonance and near-field coupling of two loop resonators was first reported by Nicola Tesla in the 1880's. See U.S. Pat. No. 645,576 to Nikola Tesla, dated 20 Mar. 1900 and titled “Systems of Transmission of Electrical Energy”. Wireless power transfer can generally be classified as radiative and non-radiative.
Radiative power transfer relies on the high-frequency excitation of a power source. Radiative power is emitted from an antenna and propagates through a medium, such as air, over a long distance, that is, many times larger than the dimension of the antenna, in the form of electromagnetic waves.
Non-radiative wireless power transfer relies on the near-field electromagnetic coupling of conductive loops, which can also be referred to as coils or conductive coils. Energy is transferred over a relatively short distance, that is, of the order of the dimension (such as the diameter) of the coupled coils.
For efficient wireless power transfer, Tesla showed that using the magnetic resonance of the coupled coils could achieve high energy efficiency. In his experiment, Tesla used a conductive coil (which is a form of air-core inductor) connected in series with a Leyden jar (which is a form of capacitor) to form a loop resonator. He excited one loop (primary coil) as the power transmitter and used a second loop resonator (secondary coil) as a power receiver. See the text titled “The man who invented the twentieth century—Nikola Tesla—Forgotten Genius of Electricity” by Robert Lomas, page 146, published by Headline in 1999 (ISBN 0 7472 6265 9).
The same use of two coupled coils for contactless energy transfer, as shown schematically in FIG. 1, has attracted much interest in the last two decades. For example, research studies on the wireless charging of batteries for electric vehicles always use a primary coil and a secondary coil. For movable industrial robots used in production lines, the use of the power cable is a nuisance. The inductive power transfer (IPT) systems for wirelessly charging industrial robots consist of a primary coil and a secondary coil coupled to each other. The coils can be in the form of concentrated windings or spiral windings. For battery charging applications, the distance between the primary and secondary coils is usually smaller than the dimension of the primary and secondary coils. The ratio of transmission distance d and the radius of the coils r is less than two, that is, d/r<3.
Such a relatively short distance is termed “short-range” wireless power transfer. For high-power applications of several kilo-Watts, the operating frequencies for wireless power transfer for electric vehicles and industrial robots are typically in the several tens of kilo-Hertz. The primary circuit and the secondary circuits are usually resonant circuits in order to maximize energy transfer, being a principle set out a century ago by Tesla. For “short-range” applications, typical energy efficiencies in the range of 80% to 95% can be achieved.
The wireless power transfer experiment carried out by a team from MIT and described in U.S. Pat. No. 7,825,543B2 to A. Karalis et al, dated November 2010 and titled “Wireless Energy Transfer”, and in the reference titled “Wireless Power Transfer via Strongly Coupled Magnetic Resonances” by Andre Kurs et al in Science, Vol. 317, 6 Jul. 2007, pages 83 to 86, is essentially based on the magnetic coupling and resonance principles laid down by Tesla. The MIT team used two coupled loop resonators, that is, one transmitter coil and one receiver coil, except that they emphasized that the distance d between the two coils is much greater than the dimension of the receiver coil, and more particularly the radius r of the circular receiver coil.
This is termed “mid-range” wireless power transfer in which the ratio of d to r is greater than 3, that is, d/r>3. In order to enable reasonable power transfer over “mid-range” distances, a high quality factor Q=ωL/Rac is needed, where ω=2πf is the angular frequency, L is the inductance, and Rac is the resistance of the loop resonator at the operating frequency f. To increase the energy efficiency, the MIT team used an operating frequency of 10 MHz. For the MIT system with a coil radius r of 30 cm and a transmission distance d of 2.4 m, that is, a ratio of d/r of 8, the MIT team reported an energy efficiency of 40%. Again, the two coupled resonators as previously proposed by Tesla were used in the MIT work.
For mid-range wireless power transfer with d>>r, it has been pointed out in both theory and practical verification that the energy efficiency between two coupled resonators is inversely proportional to d3. See the reference titled “Wireless Power Transfer Using Weakly Coupled Magnetostatic Resonators” by Jose Oscar Mur-Miranda et al in IEEE ECCE Conference, 2010, pages 4179 to 4186. This important finding indicates that the efficiency will decrease exponentially with increasing transmission distance d. This fact is confirmed by the measured energy efficiency of 40% in the reference titled “Wireless Power Transfer via Strongly Coupled Magnetic Resonances” by Andre Kurs et al in Science, Vol. 317, 6 Jul. 2007, pages 83 to 86. A typical graphical relationship of the energy efficiency versus transmission distance is shown in FIG. 2.
Besides wireless power transfer, research on metamaterials and waveguides for wave propagation and signal transfer applications has led to magneto-inductive waveguide devices, which are based on the use of a series of coupled LC loop resonators set up in a chain “with the loop planes perpendicular to an axis of wave propagation” as shown in FIGS. 3a to 3d. See the reference titled “Magneto-inductive waveguide devices” by R. Syms et al in IEE Proceedings—Microwave, Antennas Propagation, Vol. 153, No. 2, April 2006, pages 111 to 121, and the reference titled “A theory of metamaterials based on periodically loaded transmission lines: Interaction between magnetoinductive and electromagnetic waves” by R. Syms et al in Journal of Applied Physics, 97, 064909 (2005). Based on his previous work, R. Syms developed a magneto-inductive waveguide based on loop resonators printed on printed-circuit-boards (PCBs) as shown in FIG. 3d. See the reference titled “Thin-film magneto-inductive cables” by R. Syms et al in Journal of Physics D: Applied Physics, 43 (2010).
A major limitation of these waveguides, however, is that the loop resonators must be spaced a specific uniform distance apart. That is to say, there is an equal distance between each pair of adjacent loop resonators, the value of which has been specifically calculated in accordance with the characteristics of the particular loop resonators.
In the reference titled “Magneto-inductive waveguide devices” by R. Syms et al in IEE Proceedings—Microwave, Antennas Propagation, Vol. 153, No. 2, April 2006, pages 111 to 121, R. Syms also demonstrated that these waveguides can be split into more than one signal channel. An example of a 3-port signal power splitter is shown in FIG. 4, together with a graph showing the performance of such a device.
Due to the high-frequency, that is, greater than 100 MHz, and wave propagation properties in a transmission line environment, transmitted and reflected waves have to be considered together in this type of structure. Waveguides are designed for wave propagation and the operating frequencies are in the order of 100 MHz and above. Such high-frequency operation inevitably increases the AC resistance of the coils, which makes them less suitable for power transfer applications. It is also important to note that existing waveguides are essentially stationery systems. This means that all the coils are in fixed positions.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.